To study the contractile gel associated with plasma membrane of HeLa cells, we shall purify and characterize the protein components of the gel (most likely actin and a high molecular weight protein). We shall study the responses of the purified gel components to salts, temperature, sulfhydryl blockers, and cytochalasin B and determine their influence on gel formation by purified components. We shall relate the properties of gelation in vitro to the cell by studying gelation of cell extracts during the cell cycle, investigating gelation of extracts of purified plasma membranes, identifying a newly identified component of cells and plasma membrane believed to be alpha-actinin, and studying the distribution of the gel components in the plasma membrane by light and electron microscopy. We shall also investigate gelation of extracts of normal and transformed cells. To test the hypothesis that HeLa cells construct specific assemblies of microtubules from specific tubulin subunits, we shall first isolate microtubules by reconstitution in vitro, characterize the tubulin subunits, and determine whether MAP- of tau-like proteins are present and required for polymerization. We shall then isolate the colchicine mitotic apparatus and the intercellular bridge and determine if specific tubulin subunits are present in specific assemblies, and we shall also determine whether the assemblies contain MAP- or tau-like components.